Why does this happen?

Why is it that when I export an animation as SWF, the default background is white, but when I export as Movie (.MOV), it is black?

your wrong, it is in fact black by default. Go to “render view” to see that.

To get around this just add a colour card which lets you choose the colour of your background.

Isn’t having the background black a little counter-intuitive? If I draw something with the default color (which is black, too) and render to movie (MOV), I don’t see my drawing (being black on black).
First time I tried to export an animation, I thought the program had erased my drawings!

The black is not in fact a background, it represents a transparency. If you were to render with an alpha channel, you would see no background at all. If you want to add a background, you can add a colour card, which by default is set to white.

In a SWF output transparency shows up as white because that’s how SWF was originally configured, since it’s use is for the web. Most video-editing softwares will take Black as an input for transparency.